Re: [Boost-users] [foreach][range] How would BOOST_REVERSE_FOREACH be implemented?

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-01 11:34:22

Pete Bartlett wrote:
> #define REVERSE_FOREACH( x , container ) BOOST_FOREACH( x , 
> make_reverse_iterator_range( container ) )

Something like this is what I was going to suggest, but ...

> This works fine when container is an lvalue. But then co-workers, used 
> to the wonders of the original FOREACH, inevitably start to use the new 
> macro with rvalues and get a crash due to dangling references (– the 
> iterator_range’s members point to departed quantities…..)

... ouch. If the container is an rvalue, it will be copied by FOREACH, 
but it the "container" is really just a proxy (eg., an iterator pair) to 
a temporary object, then copying the proxy doesn't help you, as you've 
discovered.

<snip>

> … but all these issues are exactly the ones that BOOST_FOREACH itself 
> solves! Thus surely it is technically possible to write a 
> REVERSE_FOREACH that works with rvalues as well as lvalues, e.g. by 
> essentially copying and pasting the FOREACH implementation and replacing 
> “begin” and “end” by “rbegin” and “rend” in the vital places.

Yes.

  However
> one feels there may be a better way… has anyone had a go at creating a 
> “fully-functional” REVERSE_FOREACH and could offer me any advice? If 
> indeed the task is fairly intricate and requires re-use of foreach 
> internals, would the library author consider including something in the 
> library itself?

You're not the first to want a REVERSE_FOREACH, but this is the first 
legitimate argument I've heard for it. Could you go to 
http://svn.boost.org/trac and open a new ticket there. Make the ticket 
type "feature request" and the component "foreach". I'll look into it 
when I have a chance.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com